As methods for detecting intermolecular interactions, the surface plasmon resonance method, fluorescence resonance energy transfer method, fluorescence depolarization method, evanescent field imaging method, fluorescence correlation spectroscopy, fluorescence imaging method, enzyme linked immunosorbent assay, and the like, are known so far. Especially, the fluorescence correlation spectroscopy (FCS) has advantages of small amount of sample required for the measurement (about femtoliter), short measurement time (about 10 seconds), easiness of its automatization for HTS (in fact, an apparatus aiming at ultra HTS for performing screening of 100,000 or more test substances per day is being developed by EVOTEC), and therefore it is a superior detection system (Kaneshiro M., Tanpakushitsu, Kakusan, Koso (Protein, Nucleic acid and Enzyme), 44:1431–1438, 1999). Further, the fluorescence cross-correlation spectroscopy (FCCS) utilizing two kinds of fluorescent dyes enables detection of an interaction between molecules of similar sizes, which is difficult by FCS utilizing one kind of fluorescent dye, and its application to HTS of protein interaction is expected. However, no example of successful detection of protein interaction using FCCS has not been known so far.
In general, in a detection system for protein interaction, a protein needs to be modified with a tag for immobilization and a probe such as fluorescent dye. The inventors of the present invention previously proposed a method of modifying a C-terminal of protein in a translation system using a nucleic acid derivative such as puromycin (Japanese Patent Laid-open Application (Kokai) Nos. 11-322781 and 2000-139468). Although this method has advantages, that is, it hardly degrades functions of the protein, compared with conventional chemical modification methods or fluorescent protein fusing methods, it still has aspects to be improved, for example, the small yield of the modified protein, high cost for the preparation of a large amount of the modified protein mainly due to use of cell-free translation system. In particular, when it is used for FCCS, which is most excellent as a detection system of HTS, it is indispensable to improve the yield for practical use in analysis of genomic functions and so forth, because purification degree of fluorescence-modified protein is critical.